1. Field of the Invention
This invention relates in general to liquid crystal light valves, and in particular to liquid crystal light valves which modulate a readout optical beam in response to an input electron beam pattern.
2. Description of the Related Art
Liquid cystal light valves (LCLVs) generally employ twisted nematic liquid crystal cells in which the liquid crystal molecules are ordered such that plane-polarized light incident on the cell is twisted 45.degree. entering and untwisted 45.degree. after reflection and on exiting. When sufficient voltages are applied to particular areas of the cell, the molecular axes of some of the liquid crystal molecules in these areas orient themselves parallel to the applied electric field. Plane polarized light becomes elliptically polarized in these areas of the cell, while reflected light is unchanged in the areas of the liquid crystal to which no field is applied. An analyser transmits a portion of the elliptically polarized light and reflects the unmodulated plane polarized light. Optical images can thus be produced by the spatial voltage pattern applied to the device.
Past implementations of LCLVs have included devices which are laser-addressed, and devices which are photo-conductor-coupled to a cathode ray tube (CRT) or other light source. Laser-addressed systems, although generally offering high resolution, use generally complex optical systems for both addressing and projection. Photoconductor-coupled devices, in addition to the addressing optics, have used either multilayer construction to isolate the projection and addressing light, or are limited in terms of the wavelength of the addressing/projection light.
An electron/beam addressed LCLV has the potential to eliminate prior problems associated with photoconductors and light blocking layers. In addition, such a device could be made sufficiently fast for real-time operation. A current electron-beam addressed LCLV is a Tektronix Corp. device described in an article by Duane A. Haven, "Electron-Beam Addressed Liquid-Crystal Light Valve", IEEE Transactions on Electron Devices, Vol. ED-30, No. 5, May 1983, pages 489-492. This article describes an LCLV device in which electrons from a writing gun impinge upon a target electrode, and are subsequently erased from the target by means of flood guns. The flood guns recharge the target electrode to a controlled uniform potential. One problem associated with this device is that flood guns are not uniform, and produce a non-uniform distribution of electrons on the target electrode. This in turn produces an image that is not uniform. The Tektronix device requires the use of an expensive transmissive CRT, and its optically transmissive nature limits the choice of materials that can be used in connection with the LCLV. Some materials which might otherwise be suitable are birefringent and can distort light, and therefore are not suitable. It would also be desirable to increase the sensitivity of the device.
A related device is disclosed in a pending U.S. patent application by the present inventors, Ser. No. 927,580, filed Nov, 6, 1986, "Electron Beam Addressed Liquid Crystal Light Valve", assigned to Hughes Aircraft Company, the assignee of the present invention. In the related device the electron beam is directed against a partially conductive layer mounted on an electrically resistive membrane, which in turn electrically communicates with the liquid crystal layer. A conductive matrix, preferably in the form of a metallic grid, is disposed on the electron beam side of the partially conductive layer and divides it into a series of pixels. A voltage of one polarity is induced across the corresponding location of the liquid crystal layer when the electron beam impinges upon a particular pixel in the partially conductive layer. Charge then dissipates from the pixel into the surrounding grid at a rate which is fast enough to substantially discharge the pixel during the interval between electron beam scans, but slow enough to produce an image for a readout optical beam. An appropriate circuit is provided to dissipate the charge from the conductive matrix. The resistances and capacitances of the liquid crystal layer, the membrane and the partially conductive layer are selected to produce a substantially AC voltage across the liquid crystal during the conductive layer discharge following each electron beam scan. The pixels are thus automatically erased without the need for flood guns.
While the light valve of Ser. No. 927,580 represents a substantial improvement over prior devices, presently available manufacturing techniques are not optimum in that the metallic grid is fabricated by means of photolithographic techniques. Photolithography can be difficult, and requires expensive equipment. Also, since the device is designed to operate primarily in a reflective mode, a separate mirror must be provided to reflect a readout beam back through the liquid crystal layer.